Human metapneumovirus (hMPV) is a recently discovered paramyxovirus that is a major cause of lower respiratory tract infection. hMPV causes repeat infections in children and adults and severe disease in older patients and individuals with underlying medical conditions. Mechanisms of hMPV immunity and pathogenesis are poorly understood and there are no licensed vaccines or therapeutics. The limited information available indicates that there are fundamental differences between hMPV and other paramyxoviruses, including respiratory syncytial virus. Accordingly, the central objective of the proposed research is to elucidate mechanisms of immunity to hMPV and define the contribution of host immune responses to disease. We propose three complementary specific aims to elucidate mechanisms of hMPV immunity and pathogenesis. In Specific Aim 1, the role of antibody in protection and disease will be defined. B cell-deficient mice will be infected with hMPV and viral growth and lung pathology assessed following secondary infection. Passive serum transfer experiments will determine whether antibody is required for protection. The role of antibody- dependent cellular cytotoxicity in immunity and disease will be determined using NK cell-deficient mice. In Specific Aim 2, the capacity of hMPV fusion (F) protein to mediate protection against reinfection will be determined. Mice will be immunized with F protein and challenged with engineered viruses encoding divergent F proteins. Immunized and challenged mice will be evaluated for respiratory disease and aberrant immune responses. These experiments will test the hypotheses that hMPV F protein vaccines will not be associated with enhanced disease and that hMPV F protein is capable of inducing broadly protective immunity. In Specific Aim 3, the contribution of T cells to hMPV immunity and pathogenesis will be defined. Naive T cell-deficient mice will be infected with hMPV, and viral replication and lung pathology will be assessed. T-cell-deficient mice will be inoculated with hMPV, allowed to clear infection, and rechallenged to determine the contribution of T cells to protection against secondary infection and lung pathology. CTL epitope vaccination and adoptive transfer of cloned MHC class I-restricted epitope-specific CD8+ T cell lines will be used to determine whether virus-specific CD8+ T cells contribute to pathology. Results of the experiments proposed in this application will reveal mechanisms by which host immunity can lead to both resolution of infection and immunopathogenesis. These findings will advance knowledge of hMPV immunopathology, guide the development of hMPV therapeutics and vaccines, and have broad applicability to other respiratory viruses.